This invention relates to an air-to-fuel sensor for measuring a ratio of air to fuel, i.e., an air-to-fuel ratio in exhaust gas of an internal combustion engine, as an example.
As the air-to-fuel ratio sensing element, a zirconia-oxygen concentration cell sensor has so far been used. This sensor detects the state of combustion of the fuel at its theoretical air-to-fuel ratio in utilization of the fact that an output voltage abruptly varies at a theoretical air-to-fuel ratio point. For instance, the sensor is utilized in a system which controls the internal combustion engine for automobile in such a manner that it may be operated at the theoretical air-to-fuel ratio. With the above-mentioned oxygen sensor, however, there is no substantial change in the electromotive force in a rich atmosphere with the consequence that precise measurement of the air-to-fuel ratio at the rich side is impossible. Therefore, with a view to maintaining a substantially constant rich atmosphere, various devices have been provided at the side of the air intake system to effect the open control. With such control systems, however, there exist disadvantages such that the air-to-fuel ratio control becomes costly and the high precision control cannot be done.
Also, as the sensor for detecting the entire range of the air-to-fuel ratio, there have been proposed various sensors as described in Japanese Patent Publications No. 34077/1978 and No. 49860/1982, although none of them has yet attained a stage of being practically useful owing to technical difficulty involved. In the above-mentioned Japanese Patent Publication No. 34077/1978, there is described an oxygen sensor of a type, in which non-catalytic electrodes made of a material such as gold, silver, etc. are used as the measuring electrodes to be positioned in the exhaust gas in a zirconia tube, for measuring the air-to-fuel ratio at the rich side from the theoretical air-to-fuel ratio point. However, even the above-mentioned electrodes have the catalytic function, on account of which the gas adsorption phenomenon takes place, the reproducibility of the output voltage is poor, and, further, durability of the electrode in the gas at a high temperature and a high speed is poor, hence the sensor cannot be used practically. Furthermore, the Japanese Patent Publication No. 49860/1982 describes a method, by which the air-to-fuel ratio at both rich and lean sides can be measured. However, even this method has various disadvantages such that the current value in use is extremely low, which requires care in the electrical treatment, the art of manufacturing is difficult, and durability and response of the electrode are poor in the gas atmosphere at a high temperature and a high speed, or various other disadvantages.